The invention relates to centrifugal compressors and, more particularly, to structure in a multistage centrifugal compressor that serves to minimize or eliminate regions of reverse or separated flow, resulting in improved compressor performance.
The return passage in a multistage centrifugal compressor normally consists of a number of similar vanes or airfoils of similar geometry. For certain flow conditions, these vanes are required to turn the flow significantly, possibly resulting in flow separation in the return vanes and degradation of compressor performance.
In low flow coefficient multistage compressor designs, the flow angle going into the return vanes can be very low due to high tangential components of flow. Regardless, the vanes must eliminate the tangential component of velocity before the flow goes into the following stage. Diffuser ratios also tend to be low.
The elimination of the tangential or whirl component must therefore take place over a relatively short distance. Because of the high tangential component, this results in turning the flow from a mostly tangential flow to a radial flow in a very short distance. These requirements can lead to flow separation and performance degradation.